BCL6 is the most commonly involved oncogene in diffuse large B-cell lymphomas (DLBCL). In normal development BCL6 is required for B-cells to enter the germinal center stage of differentiation and undergo immunoglobulin affinity maturation. From the mechanistic standpoint BCL6 is a transcriptional repressor of the BTB-Zinc finger family of proteins. BCL6 represses target genes to mediate its biological effects through recruitment of corepressor proteins. In its initial five years this proposal revealed new insights into the transcriptional and biological actions of BCL6. Some highlights include the discovery that BCL6 attenuates DNA damage sensing in normal and malignant B-cells by directly repressing the ATR gene, that BCL6 can biochemically compartmentalize target genes with distinct biological functions so that they can be signaled to independently by environmental stimuli, and that recruitment of the SMRT, N-CoR and BCoR corepressors to the BCL6 BTB domain is required for repression of key checkpoint genes and the survival of DLBCL cells. This latter observation led us to develop a BCL6 inhibitor drug (as per Aim 1 and 3 of the original proposal) that is now in the process of moving to clinical trials. The current version of this proposal will build on some of these leads to more profoundly explore the mechanistic basis of transcriptional programming in normal and malignant B-cells. Specifically, we hypothesize that BCL6 controls many different biological pathways, several of which vary between normal and malignant B-cells, through biochemically distinct mechanisms. We predict that BCL6 can compartmentalize genes by either introducing a specific combination of histone modifications into their promoters alone or in combination with allied transcription factors, or by physically transporting different genes to discrete transcriptional repressor domains in the nuclei of B-cells. We will use a combination of advanced genomics tools to explore these questions. Finally we predict that many of these functionally distinct transcriptional programs regulated by BCL6 through different corepressors play important roles in maintaining various facets of the malignant lymphoma phenotype. We will explore their contribution by systematically depleting these corepressors and studying the biological impact on lymphoma cells in vitro and in vivo. Gene expression profiling will be used to identify the signatures associated with corepressor loss of function. We will submit these signatures to connectivity mapping in order to identify drugs that mimic depletion of each corepressor and will study their potential use as anti-lymphoma agents.